Due to recent increase in interest in environmental issues, the method for reducing the used amount of raw materials derived from petroleum resources have been investigated in various technical fields. As to tires currently on the market, a half or more of the total weight of a tire is composed of raw materials derived from petroleum resources. For example, since general tires for passenger cars contain such as synthetic rubber in about 20% by mass, carbon black in about 20% by mass, softeners and synthetic fibers, about 50% by mass or more of a whole tire is composed of raw materials derived from petroleum resources. Therefore, there is a demand for development of rubbers for tires prepared using raw materials derived from non-petroleum resources, the tires satisfying required properties equal to or higher than those required when raw materials derived from petroleum resources are used. For example, as to sidewall rubbers of tires, it is required to reduce rolling resistance and secure satisfactory flex crack growth resistance while maintaining physical properties for obtaining desired durability.
Japanese Patent Laying-Open No. 2003-64222 (Patent Document 1) proposes, for the purpose of providing a rubber composition which can reduce rolling resistance with decreasing consumption of petroleum oils, a rubber composition including 5 to 150 parts by weight of an inorganic filler, 0 to 30 parts by weight of a silane coupling agent and 5 to 100 parts by weight of a vegetable oil having an iodine value of 130 or less, based on 100 parts by weight of a diene rubber. However, this document proposes no rubber composition capable of exhibiting both low rolling resistance and satisfactory flex crack growth resistance when used as a sidewall rubber of a tire.
Japanese Patent Laying-Open No. 2005-53944 (Patent Document 2) proposes, for the propose of providing a rubber composition for sidewalls which can achieve low fuel consumption, a rubber composition for sidewalls containing 1 to 20 parts by weight of a composite agent composed of starch and a plasticizer based on 100 parts by weight of a rubber component composed of natural rubber and/or isoprene rubber and butadiene rubber. However, this document proposes no rubber composition which exhibits both low rolling resistance and satisfactory flex crack growth resistance.
Japanese Patent Laying-Open No. 2006-89526 (Patent Document 3) proposes, for the purpose of providing a rubber composition for tires which keeps performances needed as tire members and is improved in processability, a rubber composition for tires containing 30 parts by weight or more of silica, 5 to 15 parts by weight of calcium carbonate and 5 parts by weight or less of carbon black based on 100 parts by weight of a rubber component composed of a natural rubber and/or a modified product thereof. However, this document proposes no rubber composition capable of exhibiting both low rolling resistance and satisfactory flex crack growth resistance when used as a sidewall of a tire.    Patent Document 1: Japanese Patent Laying-Open No. 2003-64222    Patent Document 2: Japanese Patent Laying-Open No. 2005-53944    Patent Document 3: Japanese Patent Laying-Open No. 2006-89526